Surface acoustic wave (SAW) resonators are known. Such resonators are typically comprised of two sets of interdigitated, or interleaved, parallel conductors deposited on a piezoelectric substrate. FIG. 1 depicts the conductors of a SAW resonator.
SAW resonators typically operate at relatively high frequencies (e.g. at or above 100 MHz). An AC signal applied to the terminal of a SAW resonator causes an time-varying electric field to be impressed upon the piezoelectric material forming the SAW resonator. The electric field causes mechanical stress within the piezoelectric substrate in the vicinity of the terminal and a resultant acoustic wave within the substrate. The acoustic wave propagates through the substrate from one set of electrodes to an opposing set of electrodes. As the wave approaches the opposing set of electrodes the acoustic wave interacting with the piezoelectric substrate induces an electric field in the opposing electrodes. In the creation of the electric field in the opposing electrodes, part of the acoustic wave may be absorbed and part of the acoustic wave may be reflected back towards the originating electrodes. The part of the wave that is absorbed becomes the output of the SAW. The reflected wave travels back towards the originating electrode. At the input electrode part of the wave may again be reflected and part of the wave absorbed. As the wave travels toward the output electrode the reflection-absorption process may be repeated a number of times, similar to the operation of a tank circuit.
If, when the original wave arrives back at the origin, the input signal has reached its maximum, the reflected wave will be reinforced and become larger than the original transmitted wave. If, on the other hand, the input signal has not reached a maximum then the reflected signal will become attenuated at each point of reflection.
Depending upon, in part, the spacing of the electrodes within a SAW resonator and the wavelength of applied signals the reinforcement and attenuation of signals occurs at specific frequencies allowing the SAW resonator to act as a filter. Signals whose frequency reinforce reflected waves will be transmitted while other signal will be attenuated.
A SAW resonator will resonate over a relatively narrow range of frequencies due to differences in propagation paths within the SAW resonator. The propagation delay time for a signal wave transmitted between the innermost electrodes (1 and 2) may be the shortest (highest frequency) whereas the propagation delay time between the outermost electrodes (3 and 4) may be the longest (lowest frequency). The difference in frequencies determined by propagation delays may define the bandwidth of the SAW resonator. Bandwidth for SAW resonators (as with filters in general) is defined as the frequency difference between which an output signal has fallen 3 db from a maximum amplitude at a center of the resonant frequency to the 3 db point on either side.
As is known in the prior art, a single SAW resonator can be used as a bandpass filter by interconnecting the device in series between a source and a load. The arrangement, however, is only useful for very narrow bandwidths because of limitations caused by the shunt capacitance of the resonator.
For an unweighted transducer pattern (FIG. 1) (all interdigitated electrodes of equal length and equal spacing of electrodes across the transducer) the input impedance in the vicinity of the resonance frequency can be approximated by a series resonance circuit (50, 51, 52, FIG. 12) in parallel with a shunt capacitor (53). The resonant frequency of the transducer is determined by an interaction of the electrode spacing and the velocity of the acoustic wave. At the center frequency the physical separation between adjacent electrodes is equal to one-half wavelength of the acoustic wave. The Q (where Q is equal to reactive impedance over resistance) is a direct function of the number of electrode pairs in the transducer and the resonator inductance is a function of the piezoelectric coupling coefficient of the substrate material. The shunt capacitor represents the static capacitance between the electrodes in the transducer pattern.
When a constant amplitude, variable frequency input signal is applied to a SAW resonator used as a filter, a phase shift occurs in the output signal as the input signal frequency passes through the resonant frequency. The phase shift substantially equals 180 degrees from a first frequency below the resonant frequency to second frequency above the resonant frequency.
SAW filters having relatively wide bandwidths have been constructed using combinations of SAW resonators having different resonant frequencies. FIG. 2 is an example of a lattice filter using two dual port SAW elements. FIG. 3 depicts the dual port SAW resonator used in FIG. 2.
Shown in FIG. 2 is a filter comprised of a first dual port SAW resonator (12) having a first resonant frequency and a second dual port SAW resonator (14) having a second resonant frequency. The output of the first SAW resonator (12) has been connected to the output of the second SAW resonator (14) in opposite phase. The opposite phase connections of the outputs of the two SAW resonators (12 and 14) is necessary because of a phase change occuring in the output signal of one of the two SAW resonators (12 and 14) as the resonator passes through its resonant frequency. The reversal of connections on the outputs of the two SAW resonators (12 and 14) substantially avoids cancellation of signals occuring over a frequency range between the two resonant frequencies.
Operation of the SAW filter (FIG. 2) provides an output substantially as shown in FIG. 4. with the lowest resonant frequency SAW resonator providing the output shown by the curve on the left (A) and the higher frequency SAW resonator providing the output shown by the curve on the right (B). Since the outputs of the SAW resonators have been connected in opposite phase the output signals of the two SAW resonators (12 and 14) are added to provide a SAW filter (FIG. 2) output (C, FIG. 4). The SAW filter (FIG. 2) resultant (C) is a summation of the curves (A and B) having a bandwidth substantially equal to the sum of the two bandwidths of the individual SAW resonators (12 and 14).
A disadvantage of the depicted SAW filter (FIG. 2) is that (because of the opposite phase connections) it must be operated as a balanced filter. A balanced filter, as is known, must operate without grounded inputs or outputs.
Unbalanced filters are known in the art. Unbalanced filters are typically more economical to use than balanced filters because of the reduced circuit complexity associated with grounded circuitry. Because of the importance of high frequency circuits a need exists for a way to construct unbalanced filters using SAW resonators. Such a construction would have significant value in high frequency applications such as radios or other signal processing.